The present invention relates to a gas turbine system and, in particular, a system for humidifying air supplied to the compressor using measurements of upstream conditions of air and water and the known flow rate of air through the turbine.
It is well known that power output from a gas turbine may be increased by spraying a fine mist of water droplets into the air upstream of the gas turbine compressor inlet. The water has the effect of lowering the temperature of the air supplied to the compressor inlet.
The general practice of manufacturers of systems that supply a fine mist of water to an air stream of a compressor inlet is to use several water pumps of the same size, individually or together. The flow rate of water can be set equal to the flow rate of any individual pump or to the sum of the flow rates from two or more pumps. Also, the flow rate may be varied through a system of control and bypass valves. The water flow control conventionally uses the measurement of the dry-bulb temperature of the air stream at a location downstream of the water injection. This practice is unreliable because any temperature sensor that is downstream of the water injection is prone to be impacted by water droplets, which cause the temperature measurement to be skewed toward the wet-bulb temperature. Additionally, the water temperature to the misters is assumed to be the same as the wet-bulb temperature of the air, i.e., the evaporation process is assumed to occur at the wet-bulb temperature of air.
In accordance with a preferred embodiment of the present invention, a system is provided for controlling desired flow rate of water to the misters without a temperature measurement downstream of the misters. This invention measures the temperatures of inlet air and inlet water upstream of the misters and, together with the known value of the flow rate of humidified air supplied to the compressor inlet and analysis of the mass- and heat-transfer processes, the flow rate of water supplied to the misters is determined. The inaccuracies inherent in the above-described prior art process are avoided.
Particularly, the magnitude of water to be added to the air stream is determined through the use of measured dry-bulb temperature and a measure of humidity such as dew point temperature, relative humidity or wet-bulb temperature of the ambient air upstream of the misters, together with the known value of the water temperature and the air flow rate. The method involves a determination of the humidity ratio (the mass of water vapor divided by the mass of dry air) of the upstream ambient air flow from the dry-bulb and dew point temperatures. The method also involves determination of the humidity ratio of saturated air at the desired downstream conditions where the water is completely evaporated. The difference between the humidity ratio of the upstream air and the humidity ratio of the downstream saturated air, or some user-defined fraction of this difference, is used to calculate the required flow rate of misted water per unit flow rate of air. These flow rates are expressed as mass per unit of time. Finally, the total flow rate of misted water is calculated from the known flow rate of misted air and the required flow rate of water per unit flow rate of air.
More particularly, the operator specifies the relative humidity of the air entering the compressor, e.g., 99% relative humidity. The required distance and the flow rate of water that creates the desired conditions for the downstream air involves an analysis of the heat- and mass-transfer processes, which are modeled using Film Theory. Since the processes are interdependent, this calculation involves an iterative solution. The heat- and mass-transfer processes result in a process of cooling the air stream that is similar to a constant wet-bulb process. In fact, the heat- and mass-transfer process is identical to a constant wet-bulb process when the water is delivered to the air stream at the wet-bulb temperature of the air. However, the present invention enables the water to be delivered to the air at a temperature greater than or less than the wet-bulb temperature without assuming the temperatures are the same. The Film Theory solution depends on the flow rate of the water, the temperature of the misted water, the diameter distribution of the water droplets, the humidity of the inlet air, the flow rate of the inlet air, the linear velocity of the inlet air, and the temperature of the inlet air. The mass-transfer models the evaporation of water before reaching the compressor. Also, the heat-transfer process is adiabatic, which yields a corresponding energy balance. The energy balance depends on the flow rate of the inlet air, the humidity of the inlet air, the dry-bulb temperature of the inlet air, the temperature of the inlet water, the flow rate of the inlet water and the humidity of the air entering the compressor. Using the mass-transfer relationship and the energy balance given by Film Theory, the flow rate of misted water and the required distance for complete evaporation is determined from a relative humidity specified by the operator.
In a preferred embodiment hereof, the water is delivered to the air stream through a system of pumps, each of which is a different size. The second-smallest pump delivers water at twice the flow rate of the smallest pump. The third-smallest pump delivers water at twice the flow rate of the second-smallest pump. The fourth-smallest, e.g., the largest, pump has a variable speed control, such that the flow rate may be varied arbitrarily. In this way, the flow rate of water can be set to any value between the maximum capacity of the pumps and one-half the capacity. Flow rates less than one-half the maximum capacity can be set to increments of the smallest pump by turning on individual pumps.
In a preferred embodiment according to the present invention, there is provided in a control system for a water mister in a gas turbine having a compressor, an inlet to the water mister for receiving air, and an inlet to the water mister for receiving water, a method for providing humidified air to the compressor comprising the steps of determining a flow rate of water supplied to the water mister to humidify air supplied to the compressor to a predetermined relative humidity by sensing at least one property of the air in the air inlet to the water mister.
In a further preferred embodiment according to the present invention, there is provided in a control system for a water mister in a gas turbine having a compressor, an inlet to the water mister for receiving air, and an inlet to the water mister for receiving water, a method for providing humidified air to the compressor, comprising the steps of sensing at least one property of the air supplied to the water mister and controlling the flow rate of water supplied to the water mister to humidify air supplied to the compressor to a predetermined relative humidity in accordance with the sensed property.
In a still further preferred embodiment according to the present invention, there is provided an apparatus for supplying humidified air to a compressor in a gas turbine, comprising a water mister, an inlet to the water mister for receiving air, an inlet to the water mister for receiving water, sensors for determining the temperature and humidity of air in the air inlet to the water mister and the temperature of water in the water inlet to the water mister, and a water flow controller for controlling the flow to the water mister in accordance with the sensed temperatures of the air and water in the inlets to the water mister to supply humidified air to the compressor.